A waste heat recovery apparatus has heretofore been known for recovering the power by utilizing the waste heat from an internal combustion engine. The waste heat recovery apparatus has a Rankine cycle including a heater for heating an operation fluid by using the waste heat from an internal combustion engine, an expansion unit (turbine) for producing a rotational driving force by expanding the heated operation fluid, and a condenser for condensing the expanded operation fluid.
According to a Rankine cycle disclosed in, for example, JP-A-2004-60462, the rotational speed of the expansion unit is controlled so as to bring the pressure of refrigerant on a high-pressure side (pressure on the inlet side) flowing into the expansion unit into agreement with a target pressure.
Taking the pressure of the high-pressure side of the expansion unit into consideration is effective in suppressing the refrigerant flowing into the expansion unit from being transformed into gas/liquid two phases and in maintaining a suitable viscosity (suitable degree of oil film thickness) of a lubricating oil necessary for devices (e.g., sliding portion of the expansion unit) constituting the Rankine cycle. However, if the pressure of the refrigerant on a low-pressure side (pressure on the outlet side) flowing out from the expansion unit is too high, it is difficult to sufficiently maintain the pressure differential between the high-pressure side and the low-pressure side, and thus over-expansion may take place. That is, if the expansion unit is not operated so as to properly expand the refrigerant, it becomes difficult to stably and efficiently operate the Rankine cycle.
According to a Rankine cycle disclosed in JP-A-2004-108220, a pump and a generator are coupled to a main shaft of an expansion unit as a unitary structure.
A liquid or a gas (exhaust gas from a gas expansion unit or an engine) is fed to the heater for heating. The operation fluid sent from a pump is transformed into superheated vapor through the heater and is then introduced into the expansion unit. In the expansion unit, the operation fluid is adiabatically expanded, and thus the driving force is generated. The driving force operates the pump and the generator that are coupled to the expansion unit are driven by the driving force, and thus the operation of the Rankine cycle is continued, and heat energy of the exhaust gas is stored as electric energy. The operation fluid after having expanded is cooled and condensed by cooling air and the like in the condenser, and is then sucked by the pump.
At the start of the Rankine cycle, if the operation fluid flows into the expansion unit from the heat exchanger for heating, a sharp pressure differential is generated between the inlet and the outlet of the expansion unit. With this, a large force is exerted on the sliding portions of the expansion unit, and may adversely affect the durability. As such, it is difficult to stably start the Rankine cycle.